Conventionally, CIS-based thin-film solar cells which utilize, as a p-type light absorption layer, a Group I-III-VI2 compound semiconductor that has a chalcopyrite structure containing Cu, In, Ga, Se and S have been proposed. Solar cells of this type are relatively inexpensive in terms of production cost and have a high absorption coefficient in the visible to near-infrared wavelength range; therefore, such solar cells are expected to exhibit high photoelectric conversion efficiency.
A CIS-based thin-film solar cell is produced by, for example, forming a backside metal electrode layer on a substrate, subsequently forming thereon a p-type light absorption layer composed of a Group I-III-VI2 compound and further sequentially forming an n-type high-resistance buffer layer and a window layer composed of an n-type transparent conductive film.
When Ga and In are used as Group III elements forming the p-type light absorption layer, the ratio between Ga having a large energy gap and In having a smaller energy gap than Ga is adjusted, and the energy gap of the p-type light absorption layer is thereby set.
It has been reported that Ga causes defects in the p-type light absorption layer and this leads to a reduction in the photoelectric conversion efficiency.
Thus, it has been proposed to reduce the recombination centers in p-n junctions and thereby improve the photoelectric conversion efficiency by reducing the Ga concentration in the p-type light absorption layer toward the n-type high-resistance buffer layer and thus reducing defects in the surface of the p-type light absorption layer on the side of the n-type high-resistance buffer layer.
Further, with regard to the composition ratio of the Group I element(s) and the Group III element(s), it has been proposed to control the composition ratio at lower than 1.0 so as to improve the photoelectric conversion efficiency (see Patent Document 1).
It has been reported that when the ratio of a Group I element, which is a metal element such as Cu, with respect to a Group III element(s) is 1.0 or higher, the p-type polarity of the p-type light absorption layer is affected and the photoelectric conversion efficiency is consequently reduced. For example, it has been proposed to set the ratio of a Group I element(s) with respect to a Group III element(s) at 0.86 to 0.98 (see Patent Document 2 and Non-patent Document 1).